Methods for Reducing Dual Damascene Distortion

ABSTRACT

An integrated circuit structure includes a first low-k dielectric layer having a first k value, and a second low-k dielectric layer having a second k value lower than the first k value. The second low-k dielectric layer is overlying the first low-k dielectric layer. A dual damascene structure includes a via with a portion in the first low-k dielectric layer, and a metal line over and joined to the via. The metal line includes a portion in the second low-k dielectric layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/830,603, entitled “Methods for Reducing Dual Damascene Distortion,”filed on Dec. 4, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/986,855, entitled “Methods for Reducing DualDamascene Distortion,” filed on Jan. 4, 2016, now U.S. Pat. No.9,842,804 issued Dec. 12, 2017, which applications are incorporatedherein by reference.

BACKGROUND

Integrated circuit devices such as transistors are formed onsemiconductor wafers. The devices are interconnected through metal linesand vias to form functional circuits, wherein the metal lines and viasare formed in back-end-of-line processes. To reduce the parasiticcapacitance of the metal lines and vias, the metal lines and vias areformed in low-k dielectric layers.

In the formation of the metal lines and vias in a low-k dielectriclayer, the low-k dielectric layer is first etched to form trenches andvia openings. The etching of the low-k dielectric layer may involveforming a patterned hard mask over the low-k dielectric material, andusing the patterned hard mask as an etching mask to form trenches. Viaopenings are also formed to substantially aligned to the trenches. Thetrenches and the via openings are then filled with a metallic material,which may include copper. A Chemical Mechanical Polish (CMP) is thenperformed to remove excess portions of the metallic material over thelow-k dielectric layer. The remaining portions of the metallic materialare metal lines and vias.

Conventional vias may suffer from distortion, particularly when thewidth of the metal lines and vias are very small. For example, the upperportions of the vias that are slightly lower than where vias join theoverlying metal lines may suffer from kinks, which are portions that arenarrower than both overlying and underlying portions of the respectivevias.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediatestages in the formation of an interconnect structure in accordance withsome embodiments.

FIG. 12 illustrates a process flow for forming an interconnect structurein accordance with some embodiments.

FIG. 13 illustrates a FinFET underlying the interconnect structure inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Interconnect structures of integrated circuits and the methods offorming the same are provided in accordance with various exemplaryembodiments. The intermediate stages of forming the interconnectstructures are illustrated. The variations of the embodiments arediscussed. Throughout the various views and illustrative embodiments,like reference numbers are used to designate like elements.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediatestages in the formation of interconnect structures of integratedcircuits in accordance with some embodiments. The steps shown in FIGS. 1through 11 are also shown in the process flow 200 in FIG. 12.

FIG. 1 illustrates wafer 100, which includes semiconductor substrate 20and the features formed over semiconductor substrate 20. In accordancewith some embodiments of the present disclosure, semiconductor substrate20 includes crystalline silicon, crystalline germanium, silicongermanium, a III-V compound semiconductor such as GaAsP, AlInAs, AlGaAs,GaInAs, GaInP, GaInAsP, and/or the like. Semiconductor substrate 20 mayalso be a bulk silicon substrate or a Silicon-On-Insulator (SOI)substrate.

In accordance with some embodiments of the present disclosure, wafer 100is used to form a device die. In these embodiments, integrated circuitdevices 22 are formed on the top surface of semiconductor substrate 20.Exemplary integrated circuit devices 22 may include ComplementaryMetal-Oxide Semiconductor (CMOS) transistors, resistors, capacitors,diodes, or the like. The details of integrated circuit devices 22 arenot illustrated herein. In accordance with alternative embodiments,wafer 100 is used for forming interposers. In these embodiments, noactive devices such as transistors and diodes are formed on substrate20. There may (or may not) be passive devices such as capacitors,resistors, inductors, or the like formed in wafer 100. Substrate 20 mayalso be a dielectric substrate in the embodiments in which wafer 100 isan interposer wafer. Furthermore, through-vias (not shown) may be formedto penetrate through substrate 20 in order to interconnect thecomponents on the opposite sides of substrate 20.

Inter-Layer Dielectric (ILD) 24 is formed over semiconductor substrate20 and fills the space between the gate stacks of transistors (notshown) in integrated circuit devices 22. In accordance with someexemplary embodiments, ILD 24 comprises phosphosilicate glass (PSG),borosilicate glass (BSG), boron-doped phosphosilicate glass (BPSG),fluorine-doped silicate glass (FSG), tetraethyl orthosilicate (TEOS), orthe like. ILD 24 may be formed using spin coating, Flowable ChemicalVapor Deposition (FCVD), or the like. In accordance with alternativeembodiments of the present disclosure, ILD 24 is formed using adeposition method such as Plasma Enhanced Chemical Vapor Deposition(PECVD), Low Pressure Chemical Vapor Deposition (LPCVD), or the like.

As also shown in FIG. 1, etch stop layer 26 is formed over ILD 24 andintegrated circuit devices 22, if any. Etch stop layer 26 may comprisesilicon carbide, silicon nitride, silicon oxynitride, siliconcarbo-nitride, or the like. Etch stop layer 26 is formed of a materialthat has a high etching selectivity relative to the overlying dielectriclayer 30, and hence etch stop layer 26 may be used to stop the etchingof dielectric layer 30.

Contact plugs 28 are formed in ILD 24 and are used to electricallyconnect to integrated circuit devices 22. For example, contact plugs 28may include gate contact plugs that are connected to the gate electrodesof transistors (not shown) in integrated circuit devices 22 andsource/drain contact plugs that are electrically connected to thesource/drain regions of the transistors. In accordance with someembodiments of the present disclosure, contact plugs 28 are formed of amaterial selected from tungsten, aluminum, copper, titanium, tantalum,titanium nitride, tantalum nitride, alloys therefore, and/ormulti-layers thereof. The formation of contact plugs 28 may includeetching ILD 24 to form contact openings, filling a conductivematerial(s) into the contact openings until the conductive materialfills the entireties of the contact openings, and performing aplanarization (such as Chemical Mechanical Polish (CMP)) to level thetop surfaces of contact plugs 28 with the top surface of ILD 24.

Further illustrated in FIG. 1 is dielectric layer 30, which is sometimesreferred to as Inter-Metal Dielectric (IMD) layer 30 hereinafter. Inaccordance with some embodiments of the present disclosure, dielectriclayer 30 is formed of a low-k dielectric material having a dielectricconstant (k-value) lower than about 3.0, about 2.5, or even lower.Dielectric layer 30 may comprise Black Diamond (a registered trademarkof Applied Materials), an oxygen-containing and/or carbon containinglow-k dielectric material, Hydrogen SilsesQuioxane (HSQ),MethylSilsesQuioxane (MSQ), or the like.

Conductive lines 32 are formed in IMD 30. In accordance with someembodiments, conductive lines 32 include diffusion barrier layers 34 andcopper-containing material 36 over diffusion barrier layers 34.Diffusion barrier layers 34 may include titanium, titanium nitride,tantalum, tantalum nitride, or the like, and have the function ofpreventing copper in copper-containing material 36 from diffusing intoIMD 30. Conductive lines 32 are referred to as metal lines 32hereinafter.

In accordance with some embodiments of the present disclosure, metalcaps 38 are formed over metal lines 32. Metal caps 38 may also beconsidered as parts of metal lines 32 throughout the description. Insome embodiments, metal caps 38 include cobalt (Co), CoWP, CoB, tungsten(W), tantalum (Ta), nickel (Ni), molybdenum (Mo), titanium (Ti), iron(Fe), or alloys thereof. Metal caps 38 may be formed selectively usingElectroChemical Plating (ECP) or electroless plating, during which wafer100 is submerged in a plating solution. In accordance with alternativeembodiments of the present disclosure, metal caps 38 are blanket formedon metal lines 32 and dielectric layer 30, followed by an etchingprocess to remove undesirable portions. FIG. 1 illustrates that metallines 32 are in a bottom metal layer, which is the metal layerimmediately over contact plugs 28. The illustrated metal lines 32 mayalso represent the metal lines in any metal layer that is over thebottom metal layer.

FIGS. 2 through 9 illustrate the cross-sectional views of intermediatestages in a dual damascene process in accordance with some embodimentsof the present disclosure. Referring to FIG. 2, etch stop layer 40 andlow-k dielectric layers 42, 44, and 48 are formed. In accordance withsome embodiments, etch stop layer 40 is formed of silicon carbide,silicon nitride, silicon oxynitride, silicon carbo-nitride, or the like.Etch stop layer 40 is in contact with metal caps 38 and dielectric layer30.

Low-k dielectric layer 42 is formed over etch stop layer 40. Therespective step is shown as step 202 in the process flow shown in FIG.12. Low-k dielectric layer 42 may be formed of a material that isselected from the same candidate materials for forming dielectric layer30. For example, low-k dielectric layer 42 may be formed of anoxygen-containing and/or carbon-containing dielectric material, BlackDiamond, HSQ, MSQ, or the like. In accordance with some exemplaryembodiments, low-k dielectric layer 42 is formed SiCO. An exemplarycomposition of low-k dielectric layer 42 has an oxygen atomic percentagebetween about 40 percent and about 50 percent, a carbon atomicpercentage between about 10 percent and about 20 percent, and a siliconatomic percentage between about 30 percent and about 40 percent.

Low-k dielectric layer 42 may also have a low dielectric constant (low-kvalue), which may be between about 2.8 and about 3.5. Thickness T1 oflow-k dielectric layer 42 may be in the range between about 300 Å andabout 450 Å. It is appreciated that the values recited throughout thedescription are examples, and may be changed to different values.

Transition low-k dielectric layer 44 is formed over and contacting thetop surface of low-k dielectric layer 42. The respective step is shownas step 204 in the process flow shown in FIG. 12. Transition low-kdielectric layer 44 has a k value lower than the k value of low-kdielectric layer 42. The k value of low-k dielectric layer 44 is lowerthan the k value of low-k dielectric layer 42 by difference Δk, which ismore than about 0.1, or higher than about than 0.2. The k valuedifference Δk may be in the range between about 0.1 and about 0.8. Inaccordance with some embodiments of the present disclosure, the k valueof low-k dielectric layer 44 is between about 2.6 and about 2.8.

Low-k dielectric layer 44 may be formed of a material that is selectedfrom the same candidate materials for forming low-k dielectric layer 42.For example, low-k dielectric layer 42 may be formed of anoxygen-containing and/or carbon-containing dielectric material, BlackDiamond, HSQ, MSQ, or the like. In accordance with some exemplaryembodiments, low-k dielectric layer 44 is formed SiCO. An exemplarycomposition of low-k dielectric layer 44 has an oxygen atomic percentagebetween about 40 percent and about 50 percent, a carbon atomicpercentage between about 10 percent and about 16 percent, and a siliconatomic percentage between about 30 percent and about 40 percent. Inaccordance with some embodiments of the present disclosure, thickness T2of low-k dielectric layer 44 is in the range between about 30 Å andabout 150 Å.

Low-k dielectric layer 48 is formed over and contacting the top surfaceof transition low-k dielectric layer 44. The respective step is shown asstep 206 in the process flow shown in FIG. 12. Transition low-kdielectric layer 48 has a k value further lower than the k value oflow-k dielectric layer 44. The k value of low-k dielectric layer 48 islower than the k value of low-k dielectric layer 44 by difference Δk′,which is more than about 0.1. The k value difference Δk′ may be in therange between about 0.1 and about 0.3. In accordance with someembodiments of the present disclosure, the k value of low-k dielectriclayer 48 is between about 2.4 and about 2.6.

Low-k dielectric layer 48 may be formed of a material that is selectedfrom the same candidate materials for forming low-k dielectric layer 44.For example, low-k dielectric layer 48 may be formed of anoxygen-containing and/or carbon-containing dielectric material, BlackDiamond, HSQ, MSQ, or the like. In accordance with some exemplaryembodiments, low-k dielectric layer 48 is also formed SiCO. An exemplarycomposition of low-k dielectric layer 48 has an oxygen atomic percentagebetween about 40 percent and about 50 percent, a carbon atomicpercentage between about 10 percent and about 15 percent, and a siliconatomic percentage between about 35 percent and about 45 percent. Inaccordance with some embodiments of the present disclosure, thickness T3of low-k dielectric layer 48 is in the range between about 300 Å andabout 450 Å.

Low-k dielectric layers 42, 44, and 48 have increasingly lower k values.Layers 42 and 44 are also referred to in combination as layer 46, whichhas a higher k value than low-k dielectric layer 48. Also, low-kdielectric layers 42, 44, and 48 are increasingly porous, with low-kdielectric layer 44 being more porous than low-k dielectric layer 42,and low-k dielectric layer 48 being more porous than low-k dielectriclayer 44. With low-k dielectric layer 44 having a k value and a densitybetween the k values and the densities of low-k dielectric layers 42 and48, low-k dielectric layer 44 is referred to as a transition layer.Having layer 44 to be denser than layer 48 has the effect of reducingthe distortion in via, as will be discussed in subsequent paragraphs.Both the k value difference Δk′ (between the k values of low-kdielectric layer 48 and low-k dielectric layer 44) and thickness T2 ofdielectric layer 44 affect the effect of reducing the via distortion.For example, both the k value difference Δk′ and thickness T2 need to bebig enough so that the distortion 82 in via opening 58 (FIG. 7) and therespective via 64 (FIG. 11) can be eliminated. If either one of k valuedifference Δk′ and thickness T2 is too small, the effect of reducing viadistortion is sacrificed or disappears.

In accordance with some exemplary embodiments, an entirety of low-kdielectric layer 44 has a uniform or substantially uniform k value (forexample, with a variation smaller than about 0.05). At the interfacebetween low-k dielectric layer 44 and low-k dielectric layer 42, thereis an abrupt change in k values. At the interface between low-kdielectric layer 44 and low-k dielectric layer 48, there is an abruptchange in k values. In accordance with alternative embodiments, low-kdielectric layer 44 has a gradually changed values, with the upperportions of low-k dielectric layer 44 having increasingly lower k valuesthan the respective lower portions. The k values of low-k dielectriclayer 44 may also be staged to include several sub-layers, with each ofthe sub-layers having a uniform k value. The upper sub-layers have lowerk values than the respective lower sub-layers. In accordance with theseembodiments, at the interface between low-k dielectric layer 44 andlow-k dielectric layer 42, there may be, or may not be, an abrupt changein k values. At the interface between low-k dielectric layer 44 andlow-k dielectric layer 48, there may be, or may not be, an abrupt changein k values. The change in low-k dielectric layer 44 may also becontinuous, with no abrupt change of k-value and density betweenneighboring portions in low-k dielectric layer 44.

In accordance with some embodiments of the present disclosure, theformation of each of low-k dielectric layers 42, 44, and 48 includesdepositing a porogen-containing dielectric base material (such as SiOC)and a porogen. The formation of low-k dielectric layers 42, 44, and 48may be performed in a same process chamber, wherein the precursors forforming the base materials (such as SiOC) of layers 42, 44, and 48 maybe similar from each other. For example, the formation of layers 42, 44,and 48 may use same precursors, and the processes for forming layer 42may be transitioned to the formation of layer 44 (and 48) by adjustingthe amount of precursors (such as flow rates) without breaking vacuum ofthe respective chamber. Layers 42, 44, and 48 may have the same elements(such as Si, O, and C), and the percentages of the elements may besubstantially equal to each other (for example, with differences smallerthan 5 percent of the respective value). However, the porogen introducedwhen forming low-k dielectric layer 44 is more than the porogenintroduced when forming low-k dielectric layer 42, and the porogenintroduced when forming low-k dielectric layer 48 is further more thanthe porogen introduced when forming low-k dielectric layer 44. Toachieve continuously changed k value and porosity in transition layer44, the precursors may be continuously adjusted with the proceeding ofthe deposition of layer 44.

By driving out the porogen in layers 42, 44, and 48, the remaining low-kdielectric layers 42, 44, and 48 become porous and have low k values.The porogen in low-k dielectric layers 42, 44, and 48 may be driven outin a same thermal curing process. Alternatively, the formation of eachof layers 42, 44, and 48 is followed by a curing process for driving outthe porogen. In the resulting structure, the composition of layers 42,44, and 48 may be essentially the same as each other (or slightlydifferent from each other), except the porosities are different, withlayer 48 having the highest porosity, and layer 42 having the lowestporosity.

In accordance with some embodiments, instead of forming low-k dielectriclayer 42 and transition layer 44 having different k values, a singlelow-k dielectric layer 46 is formed, wherein the single low-k dielectriclayer 46 is formed at the same location layers 42 and 46 are formed.Low-k dielectric layer 46 is a homogenous material, which has a uniformk value, and has a uniform composition. Alternatively stated, thisembodiment may be considered as the transition layer 44 is eliminated,and low-k dielectric layer 42 contacts the overlying low-k dielectriclayer 48. The entire low-k dielectric layer 46 may have the propertiessame as layer 42 in the previous embodiment. For example, an exemplarycomposition of low-k dielectric layer 46 may include an oxygen atomicpercentage between about 40 percent and about 50 percent, a carbonatomic percentage between about 10 percent and about 20 percent, and asilicon atomic percentage between about 30 percent and about 40 percent.Low-k dielectric layer 46 in these embodiments may also have a low-kvalue, which may be between about 2.8 and about 3.5.

Referring to FIG. 3, photo resist 50 is applied over low-k dielectriclayer 48, and is patterned in a lithography process. The respective stepis shown as step 208 in the process flow shown in FIG. 12. Next, asshown in FIG. 4, low-k dielectric layer 48 is etched using the patternedphoto resist 50 as an etching mask, and hence trenches 52A and 52B areformed. The respective step is shown as step 210 in the process flowshown in FIG. 12. In accordance with some embodiments, the etching isperformed using a time mode, so that the etching is controlled to stopon the top surface of low-k dielectric layer 44.

In accordance with some embodiments of the present disclosure, trenches52A and 52B stop at the top surface of low-k dielectric layer 44. Inaccordance with alternative embodiments, trenches 52A and 52B extendinto low-k dielectric layer 44, and stop at an intermediate levelbetween the top surface and the bottom surface of low-k dielectric layer44. In accordance with yet alternative embodiments of the presentdisclosure, trenches 52A and 52B penetrate through low-k dielectriclayer 44, and stop at the top surface of, or extend into, low-kdielectric layer 42. Dashed lines 54 schematically illustrate thepositions of the bottom surfaces of trenches 52A and 52B in accordancewith various embodiments. After the etching, photo resist 50 is removed.

Referring to FIG. 5, photo resist 56 is applied over low-k dielectriclayer 48 and extends into trenches 52A and 52B. Photo resist 56 ispatterned in a lithography process. The respective step is shown as step212 in the process flow shown in FIG. 12. Next, as shown in FIG. 6,low-k dielectric layers 44 and 42 are etched using the patterned photoresist 56 as an etching mask, and hence via opening 58 is formed. Therespective step is shown as step 214 in the process flow shown in FIG.12. Etch stop layer 40 is then etched, exposing the underlyingconductive material such as metal cap 38. In a subsequent step, photoresist 56 is removed, for example, in an ashing step, resulting in thestructure shown in FIG. 7.

FIGS. 8 and 9 illustrate the formation of via 64 and conductive lines 66(including 66A and 66B). The respective step is shown as step 216 in theprocess flow shown in FIG. 12. Referring to FIG. 8, diffusion barrierlayer 60 is deposited, and copper-containing material 62 is formed overdiffusion barrier layer 60. Diffusion barrier layers 60 may includetitanium, titanium nitride, tantalum, tantalum nitride, or the like.Copper-containing material 62 may include copper or a copper alloy.

In a subsequent step, as shown in FIG. 9, a planarization such as CMP isperformed to remove excess portions of diffusion barrier layer 60 andcopper-containing material 62. The remaining portions of diffusionbarrier layer 60 and copper-containing material 62 form via 64 and metallines 66A and 66B. The bottom surfaces of metal lines 66A and 66B may beat one of several possible levels, for example, substantially level withthe top surface of transition low-k dielectric layer 44. Alternatively,the bottom surfaces of metal lines 66A and 66B may be between the topsurface and bottom surface of transition low-k dielectric layer 44, atthe top surface of low-k dielectric layer 42, or extend into low-kdielectric layer 42, as illustrated by dashed lines 54.

FIGS. 10 and 11 illustrate the formation of additional low-k dielectriclayers, metal lines, and vias over low-k dielectric layer 48. Therespective step is shown as step 218 in the process flow shown in FIG.12. For example, as shown in FIG. 10, etch stop layer 68 is deposited,followed by the formation of low-k dielectric layers 70, 72 (or layer74), and 76. The materials, properties, and the formation processes oflayers 68, 70, 72, and 76 may be essentially the same as that of layers40, 42, 44, and 48, respectively, and hence the details are not repeatedherein. In subsequent steps, as shown in FIG. 11, via 78 and metal lines80 are formed, similar to the formation of via 64 and metal lines 66,respectively.

In accordance with some embodiments, low-k dielectric layer 72, which isa transition layer, is formed between denser low-k dielectric layer 70and more porous low-k dielectric layer 76. In accordance withalternative embodiments, low-k dielectric layer 72 is not formed, andporous low-k dielectric layer 76 contacts denser low-k dielectric layer70 directly.

The dual damascene structures in accordance with some embodiments of thepresent disclosure may be formed over, and electrically coupled to,various types of devices including, and not limited to, planarField-Effect Transistors (FETs), Fin Field-Effect Transistors (FinFETs),resistors, capacitors, and the like. For example, FIG. 13 illustrates across-sectional view illustrating that dielectric layers 46, in whichvia 64 is located, is formed over FinFET 94, which is a part ofintegrated circuit devices 22 as shown in FIG. 11.

In accordance with some embodiments of the present disclosure, FinFET 78includes semiconductor fin 81, gate dielectric 83, and gate electrode84. Semiconductor fin 81 is over top surfaces of adjacent Shallow TrenchIsolation (STI) regions 92. STI regions 92 may also include someportions (un-illustrated) forming a ring with the illustrated portions.The un-illustrated portions are not in the same plane as the illustratedportions, and are not illustrated since they are not in the same planeas the illustrated portion of semiconductor fin 81.

Gate dielectric 83 and gate electrode 84 are on the sidewalls and a topsurface of a middle portion of gate dielectric 83 and gate electrode 84.Source and drain regions 86 are on the opposite sides of gate dielectric83 and gate electrode 84. Source/drain silicide regions 88 are on thesurfaces of source and drain region 86. Contact plugs 28 are formed toconnect to source/drain silicide regions 88, and to electrically coupleto gate electrode 84.

The embodiments of the present disclosure have some advantageousfeatures. By forming a dense low-k layer that is denser than theoverlying more porous low-k dielectric layer, the distortion of vias iseliminated or reduced. For example, FIG. 7 illustrates an exemplarydistortion, wherein lines 82 represent the sidewalls of via opening 58when the distortion occurs. It is observed that when the distortionoccurs, an upper portion of the via opening may be narrower than theoverlying portion and the underlying portion. The distortion results inthe difficulty in the formation of diffusion barrier layer (such as 60in FIG. 8), and it is difficult to form a diffusion barrier layer havinga uniform thickness. Experiments indicated that by forming a denserlow-k dielectric layer underlying a more porous low-k dielectric layer,and by allowing the portion of via that is likely to have the distortionlocated in the top portion of the denser low-k dielectric layer, thedistortion may be eliminated or at least reduced.

In accordance with some embodiments of the present disclosure, anintegrated circuit structure includes a first low-k dielectric layerhaving a first k value, and a second low-k dielectric layer having asecond k value lower than the first k value. The second low-k dielectriclayer is overlying the first low-k dielectric layer. A dual damascenestructure includes a via with a portion in the first low-k dielectriclayer, and a metal line over and joined to the via. The metal lineincludes a portion in the second low-k dielectric layer.

In accordance with some embodiments of the present disclosure, anintegrated circuit structure includes a first low-k dielectric layerhaving a first k value, a transition layer overlying and contacting thefirst low-k dielectric layer, and a second low-k dielectric layeroverlying and contacting the transition layer. The transition layer hasa second k value lower than the first k value. The second low-kdielectric layer has a third k value lower than the second k value. Adual damascene structure includes a via having a portion in the firstlow-k dielectric layer, and a metal line over and joined to the via,wherein the metal line extends from a top surface to a bottom surface ofthe second low-k dielectric layer.

In accordance with some embodiments of the present disclosure, a methodincludes depositing a first low-k dielectric layer having a first kvalue, depositing a transition layer overlying and contacting the firstlow-k dielectric layer, and depositing a second low-k dielectric layeroverlying and contacting the transition layer. The transition layer hasa second k value lower than the first k value. The second low-kdielectric layer has a third k value lower than the second k value. Themethod further includes performing a first etching step to etch thesecond low-k dielectric layer until the transition layer is exposed toform a trench, performing a second etching step to form a via openingunderlying and joined to the trench, with the first low-k dielectriclayer etched, and filling the trench and the via to form a metal line inthe trench and a via in the via opening.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An integrated circuit structure comprising: a first low-k dielectric layer having a first k value; a transition layer overlying the first low-k dielectric layer, the transition layer having a second k value lower than the first k value; a second low-k dielectric layer overlying the transition layer, the second low-k dielectric layer having a third k value lower than the second k value; and a dual damascene structure comprising: a via in the first low-k dielectric layer and the transition layer; and a metal line in the second low-k dielectric layer, wherein the metal line is over the transition layer.
 2. The integrated circuit structure of claim 1, wherein throughout the transition layer, the second k value is uniform.
 3. The integrated circuit structure of claim 2, wherein each of the first low-k dielectric layer and the second low-k dielectric layer further has a uniform k value.
 4. The integrated circuit structure of claim 1, wherein each of the first low-k dielectric layer, the second low-k dielectric layer, and the transition layer comprises silicon, carbon, and oxygen.
 5. The integrated circuit structure of claim 1, wherein the via penetrates through the transition layer and the first low-k dielectric layer.
 6. The integrated circuit structure of claim 1, wherein the metal line is limited in the second low-k dielectric layer.
 7. The integrated circuit structure of claim 1, wherein at an interface between the transition layer and the first low-k dielectric layer, there is an abrupt difference in k values of the transition layer and the first low-k dielectric layer.
 8. The integrated circuit structure of claim 7, wherein the abrupt difference is greater than about 0.1.
 9. The integrated circuit structure of claim 1, wherein at an interface between the transition layer and the second low-k dielectric layer, there is an abrupt difference in k values of the transition layer and the second low-k dielectric layer.
 10. The integrated circuit structure of claim 9, wherein the abrupt difference is greater than about 0.1.
 11. An integrated circuit structure comprising: a first low-k dielectric layer having a first k value, wherein an entirety of the first low-k dielectric layer has a uniform first k value; a transition layer overlying and contacting the first low-k dielectric layer, wherein k values reduce from a bottom surface to a top surface of the transition layer; a second low-k dielectric layer overlying and contacting the transition layer, wherein the second low-k dielectric layer has a second k value lower than k values of the transition layer; and a dual damascene structure comprising: a via extending into both of the first low-k dielectric layer and the transition layer; and a metal line over and joining to the via, wherein an entirety of the metal line is in the second low-k dielectric layer.
 12. The integrated circuit structure of claim 11, wherein a highest k value of the transition layer is lower than the second k value of the second low-k dielectric layer by a difference greater than about 0.1.
 13. The integrated circuit structure of claim 11, wherein a lowest k value of the transition layer is higher than the uniform first k value of the first low-k dielectric layer by a difference greater than about 0.1.
 14. The integrated circuit structure of claim 11 further comprising an additional metal line in the second low-k dielectric layer, wherein an entire bottom surface of the additional metal line is in contact with a top surface of the transition layer, and the entire bottom surface continuously extends from a left edge to a right edge of the additional metal line.
 15. The integrated circuit structure of claim 11, wherein at a first interface between the transition layer and the first low-k dielectric layer, there is an abrupt difference between k values of the transition layer and the first low-k dielectric layer, and at a second interface between the transition layer and the second low-k dielectric layer, there is an abrupt difference between k values of the transition layer and the second low-k dielectric layer.
 16. An integrated circuit structure comprising: a first low-k dielectric layer having a first k value; a transition layer overlying the first low-k dielectric layer, wherein the transition layer has a second k value lower than the first k value, wherein the second k value is substantially uniform in the transition layer; a second low-k dielectric layer overlying the transition layer, the second low-k dielectric layer having a third k value lower than the second k value, and wherein the second low-k dielectric layer has a uniform k value; and a dual damascene structure comprising: a via in the first low-k dielectric layer and the transition layer; and a metal line in the second low-k dielectric layer.
 17. The integrated circuit structure of claim 16, wherein an entirety of the metal line is in the second low-k dielectric layer.
 18. The integrated circuit structure of claim 16, wherein the metal line comprises: a diffusion barrier layer comprising a bottom portion and sidewall portions over and connected to opposite ends of the bottom portion, wherein a bottom surface of the bottom portion contacts a top surface of the transition layer; and a metallic region over the bottom portion of the diffusion barrier layer.
 19. The integrated circuit structure of claim 18, wherein a top surface of the bottom portion is higher than the transition layer.
 20. The integrated circuit structure of claim 16, wherein each of the first low-k dielectric layer, the transition layer, and the second low-k dielectric layer comprises silicon, carbon, and oxy 